They will make a home just about anywhere.even right through a middle of a LPS coral.I seen them on open brain and goniopora.Personally,I would crush it with a pair of pliers and remove the worm with tweezers.
you can also use a razor knife and scrape it off the shell....again carefully with the clam closed. Are the bysial(spelling blows) attached to anything? Can you pick the clam up?